Apparatus and method for fault detection on conductors

ABSTRACT

A method of detecting for a fault on one of a plurality of adjacent, alternating current carrying conductors; the method comprising the steps of detecting the waveform of a first component of a magnetic field vector representative of the magnetic field around the plurality of conductors at a location along the conductors; detecting the waveform of a second component of the magnetic field vector; monitoring, at the location, the two waveforms for any change in the waveforms; and detecting for a fault on one of the plurality of the conductors as indicated by a change in either one or both of the waveforms.

This application is the National Stage of International Application No.PCT/AU00/00314 filed Apr. 12, 2000 and published in English under PCTArticle 21(2), which claims the benefit of U.S. Provisional ApplicationNo. 60/128,746 filed Apr. 12, 1999.

FIELD OF THE INVENTION

This invention relates broadly to a method and an apparatus fordetecting faults on any one of a plurality of adjacent conductorsutilising magnetic field measurements. The invention will be describedherein with reference to fault indicators for overland powerdistribution cables, it will be appreciated, however, that the inventiondoes have broader applications for various conductor arrangements inwhich faults may be experienced.

BACKGROUND OF THE INVENTION

Equipment for detection and location of faults on a plurality ofoverland power distribution cables involves typically the measurement ofthe magnetic field produced by the alternating currents flowing in thecables, using a single magnetic field sensing coil. Such equipmentderives a signal indicative of the average magnetic field, and this isused for the detection and location of faults. When one or more of thecables experience a fault, high currents flow in the cables, producing arapid increase in the magnetic fiend around the cables. Therefore, if anincrease is detected by the equipment, this is indicative of a faultcurrent in one or more of the cables having passed the magnetic fieldsensing coil.

However, the applicant has found that the use of a single coil has thedisadvantage that there is usually at least one set of fault currentsfor a given cable configuration that will produce a change in themagnetic field around the cables that is not detectable by the singlecoil. This is because a single coil measurement cannot be utilised tomonitor changes in the magnetic field that vary the relevant magneticfield vector at the point of the measurements in a directionperpendicular to the central axis of the coil.

SUMMARY OF THE INVENTION

In accordance with a first aspect of the present invention there isprovided a method of detecting for a fault on one of a plurality ofadjacent, alternating current carrying conductors; the method comprisingthe steps of detecting the waveform of a first component of a magneticfield vector representative of the magnetic field around the pluralityof conductors at a location along the conductors, detecting the waveformof a second ( i.e., angularly displaced) component of the magnetic fieldvector; monitoring, at the location, the two waveforms over a continuoustime interval for any changes in the waveforms; and detecting for afault on one of the plurality of the conductors as indicated by a changeover the continuous time interval in either one or both of thewaveforms.

Preferably, the method further comprises the step of determining whetherthe fault experienced is a phase to phase or a phase to earth fault asindicated by a change in both waveforms.

In one embodiment, the first and second components are at 90° withrespect to each other.

Advantageously, the first component is the horizontal component of themagnetic field vector and the second component is the vertical componentof the magnetic field vector.

In one embodiment, the step of monitoring the two waveforms comprisesthe steps of monitoring a first amplitude of the first waveform over aperiod of the alternating current, and monitoring a second amplitude ofthe second waveform over the period, and the step of detecting for afault comprises detecting for the fault as indicated by a change ineither one or both of the amplitudes.

Preferably, the step of monitoring the two waveforms comprises the stepof monitoring a phase difference between the first and second waveforms,and the step of detecting for a fault comprises detecting for the faultas indicated by a change in the phase difference.

In accordance with a second aspect of the present invention there isprovided an apparatus for detecting for a fault on one of a plurality ofadjacent, alternating current carrying conductors, the apparatuscomprising first detecting means for detecting the waveform of a firstcomponent of a magnetic field vector representing the magnetic fieldaround the plurality of conductors at a location along the conductors;second detecting means for detecting the waveform of a second (i.e.,angularly displaced) component of the magnetic field vector; monitoringmeans for monitoring the two waveforms over a continuous time intervalfor any changes in the waveforms; and a detecting unit arranged togenerate a fault indication signal depending on a change over thecontinuous time interval in either one or both of the waveforms.

Preferably, the detecting unit is further arranged to determine whetherthe fault experienced is a phase to phase or phase to earth faultdepending on reference data stored in a database of the apparatus.

In one embodiment, the monitoring unit is arranged to monitor a firstamplitude of the first waveform over a period of the alternatingcurrent, and to monitor a second amplitude of the second waveform overthe period, and the detecting unit generates a fault indication signaldepending on a change in either one or both of the amplitudes.

Preferably, the monitor unit is further arranged to monitor a phasedifference between the first and second waveforms and the detecting unitis further arranged to generate the fault indication signal depending onchanges in the phase difference.

The present invention may be more readily understood from thedescription of preferred forms of an apparatus for electricalmeasurements on conductors given below with reference to theaccompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic diagram illustrating a set of three conductorsmounted on a pole. The earth acts as a fourth conductor during earthfaults.

FIG. 2 is a graph illustrating the changes of a magnetic field vectorproduced at point A of FIG. 1 for a balanced three phase set of currentsflowing in the conductors.

FIG. 3 is a graph illustrating the changes in the magnetic field vectorfor a 50 ampere earth fault on phase 1 of the set of currents of FIG. 2.

FIG. 4 is a graph illustrating the changes of the magnetic field vectorfor a 50 ampere earth fault on phase 2 of the set of currents of FIG. 2.

FIG. 5 is a graph illustrating the changes in the magnetic field vectorfor a 50 ampere phase to phase fault on phases 1 and 3 of the set ofcurrents of FIG. 2.

FIG. 6 is a schematic diagram illustrating the use of an apparatus fordetecting faults on a plurality of conductors in accordance with oneembodiment of the present invention.

FIG. 7 is a perspective view of the apparatus in FIG. 6.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

In FIG. 1, the conductors 10, 12 and 14 carry currents i₁, i₂ and i₃,and the earth carries any imbalance. The magnetic field vector at apoint A on the pole has horizontal and vertical components B_(H) andB_(V).

At point A, the magnetic field vector follows an elliptical shape overthe period of the sinewave. The shape of the ellipse varies depending onthe amplitudes and phases of the currents i₁, i₂ and i₃.

The ellipse of magnetic fields produced by four sets of currents isshown in FIGS. 2 to 5.

Turning to FIG. 2, the graph 20 shows the ellipse of magnetic fieldsproduced by a balanced set of three phase magnetic fields produced by abalanced set of three phase currents, with amplitude equal to 5 ampere:

i ₁=5 cos(wt) i ₂=5 cos(wt−2π/3) i ₃=5 cos(wt−4π/3)

B _(xmax), 22=0.33 μT, B _(ymax), 24=1.22 μT

In FIG. 3, the graph 30 shows a phase to earth fault on phase 1.Compared to graph 20 of FIG. 2, the peak horizontal and verticalmagnetic fields 32 and 34 respectively have increased as shown below:

i ₁=51.67 cos(wt) i ₂=4.41 cos(wt+101°) i ₃=4.41 cos(wt−101°)

B _(xmax)=6.35 μT, B _(ymax)=7.42 μT

Pre-fault Fault Fault: pre-fault ratio B_(xmax), 32 0.33 μT 6.35 μT 19.2B_(ymax), 34 1.22 μT 7.42 μT 6.08

Turning now to FIG. 4, the graph 40 shows a phase to earth fault onphase 2. Compared to graph 30 of FIG. 2, the peak horizontal magneticfield 42 has increased but the peak vertical magnetic field 44 has notchanged. If one was only measuring the vertical magnetic field, thefault current would not be detected:

i ₁=4.41 cos(wt+19.1°) i ₂=51.67 cos(wt−2π/3)

i ₃=4.41 cos(wt−139.1°)

B _(xmax)=9.80 μT, B _(ymax)=1.22 μT

Pre-fault Fault Fault: pre-fault ratio B_(xmax), 42 0.33 μT 9.80 μT 29.7B_(ymax), 44 1.22 μT 1.22 μT 1.0

Turning now to FIG. 5, the graph 50 shows a phase to phase fault onphases 1 and 3. Compared to graph 30 of FIG. 2, the peak verticalmagnetic field 54 has increased but the peak horizontal magnetic field52 has not changed. If only the horizontal magnetic field is measured,the fault current would not be detected:

i ₁=50 cos(wt)+2.5 cos(wt+300°) i ₂=5 cos(wt−2π/3)

i ₃=50 cos(wt−π)+2.5 cos(wt+300°)

B _(xmax)=0.33 μT, B _(ymax)=14.08 μT

Pre-fault Fault Fault: pre-fault ratio B_(xmax), 52 0.33 μT  0.33 μT 1.0B_(ymax), 54 1.22 μT 14.08 μT 11.54

FIGS. 4 and 5 illustrate the limitations of only measuring one of thehorizontal or vertical components of the magnetic field vector.

Use of a single coil at a fixed angle to the horizontal shows similarlimitations. This is because there is usually at least one set of faultcurrents (for one of the conductor configurations in general use) thatwill produce a change in magnetic field that will not be detectable bythe single coil.

A possible solution is to mount the magnetic field sensing point A awayfrom the axis of the middle conductor. This is troublesome in practice,requiring special mounting equipment and calculation of the optimumposition for each conductor configuration.

The measurement of both vertical and horizontal magnetic fields removesthe limitations found above, and decreases the possibility of missingthe passage of fault current.

Turning now to FIG. 6, a device 100 embodying the present inventioncomprises two coils 102 and 104 to measure the horizontal and verticalcomponents 106, 108 respectively of a magnetic field vector 110indicative of the magnetic field around an arrangement of threeconductors 112, 114 and 116 at a location along the conductors.

The device 100 further comprises a detecting unit 120 for detectinganalogue voltage signals derived from the coils 102, 104. The detectingunit 120 generates signals 122 indicative of the waveforms of thecomponents 106 and 108 respectively.

In the monitoring unit 124, a fault detection signal 126 can begenerated depending on changes in the waveforms.

Changes in the waveforms can be indicative of the occurrence of faultson one or more of the conductors 110, 114 and 116, as illustrated in theexample scenarios of FIGS. 2 to 5. For example changes in the parametersmeasured for the waveforms of the components 106 and 108, such as e.g.their amplitudes, phase difference, harmonic content etc., over a periodof the alternating current in the conductors 110, 114, and 116 may beutilised. Alternatively, the waveforms of the components 106 and 108 maybe compared with reference data stored in a database (not shown).

The embodiment described above utilises two coils to measure thehorizontal and vertical components of the magnetic field vector. Thedirection of the component measured is determined by the central axis ofthe coils. It will be appreciated by a person skilled in the art thatthe present invention is not limited to the measurement of thehorizontal and vertical components, but rather any two components may bemeasured, provided that the components are angularly displaced. It isadvantageous that the angle between the components is 90°.

It will also be appreciated by a person skilled in the art that thepresent invention is not limited to the use of two coils for measuringthe respective components of the magnetic field vector. The measurementsmay be performed utilising a movable coil which can be angularlydisplaced to perform sequences of measurements for the respectivecomponents.

It will be appreciated by a person skilled in the art that numerousvariations and/or modifications may be made to the present invention asshown in the specific embodiments without departing from the spirit orscope of the invention as broadly described. The present embodimentsare, therefore, to be considered in all respects to be illustrative andnot restrictive.

In the claims that follow and in the summary of the invention, exceptwhere the context requires otherwise due to express language ornecessary implication, the word “comprising” is used in the sense of“including”, i.e. the features specified may be associated with furtherfeatures in various embodiments of the invention.

What is claimed is:
 1. A method of detecting for a fault on one of aplurality of adjacent, alternating current carrying conductors; themethod comprising the steps of: detecting the waveform of a firstcomponent of a magnetic field vector representative of the magneticfield around the plurality of conductors at a location along theconductors; detecting the waveform of a second component of the magneticfield vector; monitoring a first amplitude of the first waveform over aperiod of the alternating current; monitoring a second amplitude of thesecond waveform of the second component over the period; and detectingfor a fault on one of the plurality of the conductors as indicated by achange in either one or both of the amplitudes.
 2. A method as claimedin claim 1, further comprising the step of determining whether the faultexperienced is a phase to phase or a phase to earth fault as indicatedby the change in either one or both of the waveforms.
 3. A method asclaimed in claim 1, wherein the first and second components are at 90°with respect to each other.
 4. A method as claimed in claim 1, whereinthe first component is the horizontal component of the magnetic fieldvector and the second component is the vertical component of themagnetic field vector.
 5. A method as claimed in claim 1, wherein thestep of monitoring the two waveforms comprises the step of monitoring aphase difference,between the first and the second waveforms, and thestep of detecting for a fault comprises detecting for the fault asindicated by a change in the phase difference.
 6. An apparatus fordetecting for a fault on one of a plurality of adjacent, alternatingcurrent carrying conductors, the apparatus comprising a first detectingmeans for detecting the waveform of a first component of a magneticfield vector representing the magnetic field around the plurality ofconductors at a location along the conductors; second detecting meansfor detecting the waveform of a second component of the magnetic fieldvector; monitoring means for monitoring the two waveforms for any changein the waveforms, the monitoring means being arranged to monitor a firstamplitude of the first waveform over a period of the alternating currentand to monitor a second amplitude of the second waveform over theperiod; and a detecting unit arranged to generate a fault indicationsignal depending on a change in either one or both of the amplitudes. 7.An apparatus as claimed in claim 6, wherein the detecting unit isfurther arranged to determine whether the fault experienced is a phaseto phase or phase to earth fault depending on reference data stored in adatabase of the apparatus.
 8. An apparatus as claimed in claim 6,wherein the monitoring unit is further arranged to monitor a phasedifference between the first and second waveforms, and the detectingunit is arranged to generate the fault indication signal depending onthe phase difference.